So-called “dry fermentation” allows pourable biomasses from organic wastes, agriculture and communal garden and park areas to be methanized without transforming the materials into a pumpable, liquid substrate. It is possible to ferment biomasses having a dry substance fraction of up to 50%. This dry fermentation method is disclosed, e.g., in EP 0934998.
In “dry” fermentation, the material to be fermented is not stirred into a liquid phase as is the case in, for example, the liquid fermentation of organic wastes. Instead, the fermentation substrate introduced into the fermenter is permanently kept humid by withdrawing the percolate at the bottom of the fermenter and again spraying the percolate over the biomass. In this way, optimum living conditions for the bacteria are achieved. By recirculating the percolate, it is additionally possible to regulate the temperature and to add additives for process optimization.
The publication WO 02/06439 describes a bioreactor, or fermenter, having the form of a pre-fabricated garage that is operated along the principle of dry fermentation in a so-called batch process. Following an inoculation with previously fermented material, the fermentation substrate is filled into the fermenter with the aid of wheel loaders. A fermentation vessel that has a garage-type construction is closed by a gas-tight gate. The biomass is fermented under exclusion of air, with no further blending taking place and no additional material being supplied. The percolate that trickles out of the fermentation material is withdrawn via a drainage channel, intermediately stored in a tank, and again sprayed over the fermentation substrate for humidification. The fermentation process takes place in the mesophilic temperature range at 34-37° C. The temperature is regulated by heating the floor and wall of the fermentation vessel.
The generated biogas may be utilized in a cogeneration unit (Blockheizkraftwerk or BHKW) for the generation of electricity and heat. In order to ensure a constant, sufficient supply of biogas for the cogeneration unit, several fermentation vessels are successively operated in the dry fermentation plant. At the end of the dwell time, the fermenter volume is emptied completely and then charged anew. The fermented substrate is supplied to subsequent composting, resulting in the formation of an organic fertilizer comparable to conventional composts.
Due to the batch-type operation, the single fermenters must be shut down from time to time. For example, the biogas production must be stopped, the fermented biomass must be removed from the respective fermenter, fresh biomass must be charged into the fermenter, and the biogas production is then resumed. During startup of a freshly charged fermenter, the methane content in the generated biogas initially is so low, and the proportions of carbon dioxide and nitrogen are so high, that the direct use of the biogas in the cogeneration unit is not possible. Moreover, fresh biomass cannot be added in parallel to fermenters already running in the batch operation, as the quality of the biogas generated in the freshly charged fermenter is too poor and would have the result that the quality of the total gas flow would no longer be sufficient to use in a cogeneration unit. Depending on the size of the container and the type and quality of the biomass, it takes between five and twelve hours for the biogas generated in the freshly charged fermenter to achieve a quality that would allow its use in a cogeneration unit or that would allow it to be mixed in with the biogas from fermenters already having been operated for a longer period of time. During this startup phase of a freshly charged fermenter, the generated biogas with the lower methane content is therefore discharged directly to the atmosphere or is flared off if the methane content is higher. Thus, a portion of the generated methane is not utilized and becomes “methane slack.”
The patent applications DE 102007024911.1 and EP08156915.4 disclose initially connecting the common biogas line from a plurality of fermenters operating in parallel to a gas processing plant. Inside the gas processing plant, the methane content is raised by filtering out nitrogen and carbon dioxide. As the gas processing plant is positioned along the common biogas line, it is only possible to improve the gas quality of the mixed biogas in the common biogas line. But it is not possible to improve the quality of the biogas generated directly in individual fermenters. As a consequence, it is not possible to reduce the methane slack occurring particularly during the startup of freshly charged fermenters.
The publication DE10047264A1 describes a method of separating biogas in a gas treatment unit into first partial flow having a high methane content and second partial flow having a low methane content. The first partial flow having the high methane content is supplied to a gas engine as fuel, and the second partial flow having the low methane content is recirculated into the biogas generation process.
It is therefore an object of the present invention to reduce the methane slack that is discharged or burned during the startup of a fermenter.